The invention relates to the general field of photolithography with particular reference to optical proximity effects and methods to overcome them.
Photolithography has been used, almost exclusively and for many years, to form the various components that make up integrated circuits. The continued increase in the density of components that can be placed on a chip has been largely due to advances in photolithography associated with using radiation of ever decreasing wavelengths. As long as the minimum size (critical dimension) of the components was greater than the wavelength of the radiation being used to expose the photoresist, advances in the art did not require any changes in the masks used other than to reduce the sizes of the components.
At a certain point in time the critical dimensions got to be less than about half the wavelength of the radiation being used, so radiation of lower wavelength had to be substituted. Eventually, critical dimensions reached, and then went below, the lower limit of optical lithography where conventional optics and resists can still be used (about 180 nm). Although it has been demonstrated that X-ray lithography is capable of producing patterns whose critical dimension is one or two orders of magnitude less than that, cost considerations have continued to drive conventional (optical) lithography to seek ways to image sub-optical critical dimensions while still using optical techniques.
When the wavelength of the imaging radiation gets to be less than the critical dimension, the effects of diffraction, though always present, become prominent enough to introduce noticeable distortions into the images projected relative to their original shapes on the imaging mask. These distortions are particularly sensitive to the distances between the various features in the pattern and are therefore often referred to as xe2x80x98proximity effectsxe2x80x99. In FIG. 1a we show an example of a line segment 11 that might appear within a typical line pattern while FIG. 1b shows how that line would end up in a photoresist mask formed using radiation less than about half the CD. The line image 12 can be seen to be shortened at both ends as well as being distorted because of rounding of the edges.
One effective way to deal with these has been to introduce distortions, known as serifs, into the original imaging pattern that compensate for the distortions that are introduced by the diffraction process. Serifs usually take the form of small squares (such as 23 in FIG. 2) that are superimposed on the corners where lines, such as 21, (in the pattern that is to be imaged) terminate. They may be custom generated through a lengthy process in which Fresnel diffraction theory is applied to compute the size, shape, and position of each serif individually or an average serif size may be preselected and then applied to all line ends in the pattern. The latter approach, while definitely less time consuming as far as computation is concerned, still requires each serif to be individually formed through electron beam imaging of the reticle pattern.
A general problem associated with the use of serifs (regardless of how they were generated) is illustrated in FIG. 3. This is the case of when two lines, such as 21a and 21b, have their ends close together so that serifs 23, routinely superimposed on the end corners, come so close to each other as to be unresolvable by the optical system. The result is the merger of adjacent serifs to form short circuiting bridges between the two lines 21a and 21b. 
The present invention overcomes these problems by abandoning the use of serifs entirely, as will be described below. A routine search of the prior art was conducted but no references teaching the exact method of the present invention were found. Several references of interest were, however, encountered. For example, Kim (U.S. Pat No. 5,804,339) describes a process for manufacturing a photo mask (as opposed to an etch mask) using an electron beam. Conventional serif geometry is used to deal with corner rounding effects of the type associated with electron beams, namely the result of electron back-scattering (as opposed to the diffraction effects encountered during optical lithography). To minimize the effects of such back-scattering, Kim first does a normal exposure of the resist followed by a very brief second exposure through a mask that contains the serifs. The second exposure is between 0.75 and 2.25% of the first exposure. The method does not solve electron beam proximity effect problems that occur when two lines have ends close to each other (as discussed above for the optical case).
A similar method to Kim""s is described by Takahashi (U.S. Pat No. 5,082,762), except that the exposure medium is an ion, rather than an electron, beam. Exposure to the beam is done in two steps, first the main mask and then the serifs.
Sporon-Fiedler et al. (U.S. Pat No. 5,208,124) describe a process intended for use with positive resists. For a pattern containing a mix of isolated and densely packed lines, lines are made narrower than their intended widths, the more isolated the line the greater the degree of narrowing. Proximity effects then have the opposite effect (i.e. the more densely packed the line the greater the shrinkage) so all widths come out the same.
Bradshaw (U.S. Pat No. 5,112,724) also teaches a two exposure process but the same mask is used for both exposures and no serifs are involved. Borodovsky (U.S. Pat No. 5,532,090) shows how fine holes can be formed by first partly exposing a pattern for holes that are larger than desired and then doing a second, reduced intensity, exposure using an aligned hole pattern having the desired dimension.
It has been an object of the present invention to provide a process for producing images by photolithography at wavelengths close to or below the CD.
Another object of the invention has been that said process be simple to apply and be equally effective for densely packed line patterns as well as isolated lines.
A further object of the invention has been that said process be suitable for hole patterns.
These objects have been achieved by providing a process based on a two reticle approach. The first, or primary, reticle contains the image that is to be transferred to the photoresist. It is used to expose the resist in the usual way to the correct dosage of light needed to optimally activate it. For a primary reticle bearing a line pattern, the second, or correction, reticle bears a pattern of rectangles which are located and dimensioned so that, when aligned relative to the primary reticle, they overlap all line ends in the pattern. The amount by which the rectangles overlap the lines is similar to the amount by which serifs (if they had been used) would have overlapped. The amount by which the rectangles extend outside the line ends is not critical (provided it is at least as large as the inside overlap amount). This property can be put to good use by allowing a single rectangle to be shared by many line ends. After the first exposure, the correction reticle is substituted for the primary reticle and, after alignment, a second, much shorter, exposure is given. The resist is then developed in the normal way, resulting in a patterned etch mask that is largely free of distortion. A similar approach applies to hole patterns except that a positive resist must be used. An important advantage of the process is that the rectangles can be generated from the original mask pattern using a simple geometric rule.